The present invention relates generally to the field of seismic exploration. More particularly, the present invention relates to a method of internal seismic cable shielding.
Seismic exploration data in the frequency band of 5 Hz to 150 Hz are presently gathered in the field using multiple twisted-wire-pair seismic cables in which the number of circuits is typically in the range from 100 to 250. Further, when acquiring seismic data, more wire pairs are typically deployed on the ground than are actually used at any one time. However, the quality of the multi-channel data is typically reduced because of crosstalk (cross-feed) between wire pairs in the cable as well as by pickup of 60 Hz and its harmonics from power lines. If 60 Hz becomes too great a problem, most seismic recording systems are equipped with notch filters that will cut this narrow band signal by 80 dB or more. However, notch filters have the undesirable property of distorting the phase of the desired seimic signals within the frequency range of about 50 to 75 Hz for a 60 Hz notch filter. This problem is not so severe when the bandwidth of interest in seismic recording ranged from 5 Hz to 50 Hz, but, as bandwidth capabilities have been expanded to higher frequencies in the range of 5 Hz to 250 Hz and above, this notch filter phase distortion effect falls in the frequency band of interest. The higher harmonics of the 60 Hz signals now also cause interference pickup problems. Because cross-talk between wire pairs within the cable is due to inductive and capacitive coupling, it also increases significantly for higher frequencies and as the length of the cable is increased.
One technique that can be used for decreasing cross-talk and power line pickup is to use shielded wire pairs (i.e., wire pairs with grounded mesh shields around each wire pair) and external cable shielding and grounding. However, shielded wire pair cable which utilizes such a technique is not normally used since it is more expensive, heavier, stiffer and more difficult to work with in the field than unshielded cable. As such, the art has heretofore lacked an appropriate method of seismic cable shielding which overcomes the foregoing limitations and effectively reduces internal cross-talk and 60 Hz pick up from external sources.